This invention relates to a communication systems and particularly to active hybrid port circuits for time division multiplex (TDM) communication systems.
The art has progressed to the extent that active electronic hybrid circuits are presently available for providing two-way transmission over a communication line via separate one-way transmit and receive channels of a common time division transmission link. One such electronic hybrid is disclosed in D. G. Medill et al., U.S. Pat. No. 3,789,154 of Jan. 29, 1974, as part of an electronic key telephone station circuit. Other active hybrid circuits are described in T. G. Lewis U.S. patent application Ser. No. 343,825, filed Mar. 22, 1973 now U.S. Pat. No. 3,828,146 to be issued Aug. 8, 1974, and D. G. Medill et al., U.S. patent application Ser. No. 343,824, filed Mar. 22, 1973, now U.S. Pat. No. 3,835,529.
Each of the foregoing hybrid circuits utilize capacitor arrangements for TDM signal sampling and holding together with operational amplifiers for conveying transmit and receive signals for a two-way station line to the one-way TDM transmit and receive buses. Feedback arrangements are used between the amplifiers to control signal cancellation required for precluding the transmit and receive signals from undesirably recirculating in the system. The degree of such signal cancellation depends substantially upon delicate impedance balance of, for example, the station line impedance and circuit components at the frequencies of operation.
Heretofore, it has proven to be a problem to maintain the impedance balances, and hence active hybrid stability and transmission quality over operational frequency ranges required by present day customers. Such imbalances and instability increase system susceptibility to impaired transmission fidelity, objectionable crosstalk, noise transients, and/or singing conditions.
In view of the foregoing, it is apparent that a need exists for means to improve impedance balance and stability arrangements in active hybrid circuits and particularly to achieve quality transmission with minimal crosstalk, singing and noise transients as a result of active hybrid circuit operations with a communication line in a TDM system.